Global Nitrous Oxide Production Determined by Oxygen Sensitivity of Nitrification and Denitrification

The ocean is estimated to contribute up to ~20% of global fluxes of atmospheric nitrous oxide (N ₂ O), an important greenhouse gas and ozone depletion agent. Marine oxygen minimum zones contribute disproportionately to this flux. To further understand the partition of nitrification and denitrification and their environmental controls on marine N ₂ O fluxes, we report new relationships between oxygen concentration and rates of N ₂ O production from nitrification and denitrification directly measured with ¹⁵ N tracers in the Eastern Tropical Pacific. Highest N ₂ O production rates occurred near the oxic-anoxic interface, where there is strong potential for N ₂ O efflux to the atmosphere. The dominant N ₂ O source in oxygen minimum zones was nitrate reduction, the rates of which were 1 to 2 orders of magnitude higher than those of ammonium oxidation. The presence of oxygen significantly inhibited the production of N ₂ O from both nitrification and denitrification. These experimental data provide new constraints to a multicomponent global ocean biogeochemical model, which yielded annual oceanic N ₂ O efflux of 1.7–4.4 Tg-N (median 2.8 Tg-N, 1 Tg = 10 ¹²  g), with denitrification contributing 20% to the oceanic flux. Thus, denitrification should be viewed as a net N ₂ O production pathway in the marine environment.